In fields where digital moving image information is handled, the information is compressed. One typical compression technique is MPEG (e.g., MPEG-1, MPEG-2, H. 264/MPEG-4 AVC). The MPEG-1 has been used in old-generation package media (e.g., a video CD). In the meantime, the MPEG-2 is used in digital television broadcasting or current-generation video package media (e.g., a DVD). Moreover, the H. 264/MPEG-4 AVC or the like is used to more efficiently compress high-quality digital moving image information such as high-definition pictures.
The MPEG-2 is used for the high-definition pictures of, for example, a digital television broadcast, but the compression efficiency of the MPEG-2 is becoming less than sufficient. For example, when high-definition pictures (the bit rate is a fixed rate of 24 Mbps) of a satellite digital broadcast are to be recorded into a single-sided single-layer DVD-R disc having a capacity of 4.7 GB, little more than twenty minutes of pictures can only be recorded. However, if the high-definition pictures are re-encoded by, for example, the H. 264/MPEG-4 AVC and thus converted (transcoded) into new high-definition pictures (the bit rate is a variable rate of about 4 Mbps in average), about two hours of high-definition pictures of the digital television broadcast can be recorded in the 4.7 GB disc.
Under these circumstances, development is proceeding in moving image encoding techniques for transcoding a first encoding stream (e.g., an MPEG-2 stream) into a second encoding stream (e.g., an H. 264/MPEG-4 AVC stream) (see Jpn. Pat. Appln. KOKAI Publication No. 2006-33014).
In Jpn. Pat. Appln. KOKAI Publication No. 2006-33014, transitions in the complexity of encoding and the amount of generated bits are regarded as equal in order to achieve adaptive bits allocation. That is, if a quantization parameter remains the same throughout a sequence, the transition in the generated bits of an input image stream can be regarded as the same as the transition in the encoding complexity throughout the sequence, and bits can be allocated in accordance with the encoding complexity for each frame. However, in fixed bit rate control which is used in, for example, the digital television broadcasting, the quantization parameter is changed to maintain a constant bit rate, so that the transition in the generated bits cannot always be regarded as the same as the transition in the encoding complexity. That is, it is difficult to achieve the adaptive bits allocation corresponding to the contents of an input image stream.